Long rod 306??? 400 sbc crank and chevy rods.

[red82gt]

The longer rod will allow the piston to stay down in the bore for a longer period of time and will increase piston speed without increasing stroke. I can't think of a good analogy to really explain this though. The increased piston speed is what gives the extra torque.
The reason it's not done from the factory is because the clearances are much tighter and would be difficult to keep in check on a production line. The piston also spends more time at the top of the bore so piston/valve clearance could be a problem with a healthy cam.
I also gurantee that a Chebby 400 crank would be impossible to fit in a 302 block, heck I highly doubt that the two blocks even have the same length.